1. Field of the Invention
The present invention relates to a self-acting air bearing which is used in a polygon motor mounted in an optical writing system, such as a digital copying machine, a digital facsimile, laser printer, etc., and particularly relates to a self-acting air bearing used in a polygon motor of aluminum alloy.
2. Discussion of the Background
In a polygon motor mounted in an optical writing system in a digital copying machine, as an example, a self-acting air bearing is often used in order to achieve stable and high speed copying. In this self-acting air bearing, liquid lubrication is maintained ordinarily. However, when rotating starts or stops or the machine is shocked, a shaft and a bearing directly contact each other, and serious damage such as baking is caused in some cases.
In order to prevent such a catastrophe, lubricant oil is often used in the air bearing. However, if the oil is used in a copying machine, it may be scattered into an optical writing system and writing performance can be seriously degraded, and therefore the lubricant oil is not used in the polygon motor. Thus, background solutions are that a solid lubricant is used in at least one of the shaft and the bearing, that a high abrasion resistance material is coated on surfaces of the shaft and/or bearing, or that oil leak is prevented by a magnetic liquid. More specifically, as examples of these solutions, BN composite plating is processed on a contacting surface of the bearing for lubrication (see Japanese Laid Open Patent No. 1986-112818), SiC composite plating is processed on a rotating shaft for high abrasion resistance and lubricative alumilite film is processed on a fixed shaft (see Japanese Laid Open Patent No. 1988-235719), lubricant of a perfluoropolyether (this is referred to as PFPE) group is coated on a ceramic material (Japanese Laid Open Patent No. 1992-808062), a resin including fluorine is coated on a metal for improving abrasion resistance (see Japanese Laid Open Patent No. 1989-65322) etc., and there have been bearing processing methods in which lubricity are previously added, as described above.
As first background solving methods, it is disclosed that a lubricant of low volatility is coated on at least one of the shaft and the bearing (see Japanese Laid Open Patent Nos. 1992-808062 and 1989-65322). However, in this method in which the lubricant is coated, the lubricant disappears from a rotating surface as rotation is repeated. As the typical lubricant, PFPE, PTFE, MoS.sub.2, or BN is used. However, such a lubricative material has a serious disadvantage of a low abrasion resistance, and therefore the lubrication cannot be maintained for a long time period. Furthermore, it is disclosed recently that a plating in which such a material is dispersed is used as a surface processing (see Japanese Laid Open Patent Nos. 1986-112818 and 1991-186608, etc.). However, even if such a method is used, the shaft itself has lubrication, and therefore there is a disadvantage that processability is worsened and processing accuracy, which is necessary for high speed rotation, cannot be achieved.
Moreover, in a second solving method in which a high abrasion material is coated on uppermost surfaces of the shaft and the bearing, though ceramic is coated mainly, the filming method is divided into a dry process and a wet process (see Japanese Laid Open Patent Nos. 1986-282623 and 1988-8834, etc.). However, if a small size and low weight motor is developed and aluminum alloy is used as a base material, a limit becomes recognized in filming by the dry process. First, a low temperature process is obliged because of an aluminum base and a possibility that an adhering force is lower and an adhering state is worsened and becomes high. Moreover, since a hardness difference between a filmed ceramic and base material is large, a crack can be generated easily with contact. In order to solve this, thick filming is necessary. However, the dry process does not meet cost requirements. Furthermore, dry coating after plating is processed has been considered. However, this method is also disadvantageous in cost. Therefore, recently a method using an anode oxidized film is disclosed (see Japanese Laid Open Patent Nos. 1992-277317 and 1992-140509, etc.). However, this method has a serious disadvantage that reproduction is impossible. Moreover, though hardness is high, thoroughness is low, and therefore large abrasion powder is easily generated which can directly cause a lock in a high accuracy motor where clearance between a shaft and a bearing is only around 10 .mu.m. In a third solving method, though magnetic liquid is used, fear that oil leak is generated and pollution of a polygon mirror is generated is not denied.